1. Field of the Invention
The present invention is directed to fuel cells and more particularly to fuel cells having electrolyte reservoirs.
2. Description of the Prior Art
Fuel cells used to convert the latent chemical energy of a fuel directly into electricity are well-known in the art. For example, see U.S. Pat. No. 4,463,068. Such cells may be based on a variety of electrochemical reactions. One well-known reaction is based on using hydrogen as a fuel which reacts with oxygen to generate electricity.
One common form for constructing a hydrogen oxygen cell is a laminated structure wherein an anode electrode and a cathode electrode are spaced apart by a porous layer of material which holds an electrolyte such as concentrated phosphoric acid. The hydrogen is guided by passageways behind the active region of the anode and the oxygen is guided by passageways behind the active region of the cathode. Both the anode and the cathode have a catalyst, such as platinum, deposited thereon.
At the anode, the hydrogen gas dissociates into hydrogen ions plus electrons in the presence of the catalyst. The hydrogen ions migrate through the electrolyte to the cathode in a process constituting ionic current transport while the electrons travel through an external circuit to the cathode. At the cathode, the hydrogen ions, electrons, and molecules of oxygen combine to produce water.
It is known that fuel cells using phosphoric acid as an electrolyte have insufficient control volumes between the electrodes and within the matrix to compensate for acid volume expansion or upsets. Such expansion can result from operational changes of pressure, temperature, and utilization factors. The nature of phosphoric acid is such that it stays in equilibrium with the water partial pressure to which it is exposed. Therefore, any changes in the water partial pressure due to operational changes will cause water to evaporate from the acid, concentrating it and causing a volume loss, or will cause water to be absorbed by the acid, diluting it and causing a volume increase. The phosphate radical has a very low vapor pressure so its mass is constant through the postulated acid volume upsets.
Further, acid additions to cells at assembly and during lifetime may further upset the acid volume due to differing conditions between those at which acid is added and stack operating conditions. If acid is added such that an excess of the phosphate radical is taken on by the fuel cell, then at operation, excess water will be absorbed causing a further volume expansion.
Another variable in determining the right amount of acid in the fuel cell is the dimensional variation between cells in a stack. This variation can cause as much as ten percent variation in the acid volume required.
The reason that acid expansion is a problem is that the electrode catalyst layers with the matrix between them depend on gas diffusion for the process gases to reach the catalyst sites while the acid must also be present for the electrochemical reaction to occur. If there is too much acid, flooding of the electrodes can occur causing a loss o gas diffusion capability and therefore a loss of cell performance.
The prior art has attempted to solve this generic problem by providing an inventory control volume capable of absorbing electrolyte during periods of electrolyte expansion and desorbing electrolyte during periods of electrolyte contraction. One fuel cell having such a reservoir is disclosed in U.S. Pat. No. 4,038,463. An electrolyte reservoir layer, which is porous and hydrophilic to the electrolyte, is disposed behind and adjacent to one of the catalyst layers of the fuel cell. In one embodiment, the reservoir includes impregnations of hydrophobic material to provide reactant gas passages through the reservoir layer to the catalyst layer. The impregnations of hydrophobic material are designed to provide good distribution of the reactant gas into the catalyst layer without consuming a large volume of the reservoir. Additionally, the reservoir layer includes impregnations of a material similar to the electrolyte retaining matrix material to improve electrolyte transfer from the matrix into the reservoir. Although such a construction does provide for an electrolyte inventory control volume, it substantially complicates the production of the fuel cell. It is difficult to manufacture a reservoir layer having such discrete hydrophobic and hydrophilic areas together with areas impregnated with other materials. Such difficult manufacturing procedures also increase the cost of the fuel cell.
Another attempt at providing an electrolyte reservoir is disclosed in U.S. Pat. No. 4,035,551. In that reference an electrolyte reservoir layer is disposed behind and adjacent to one of the catalyst layers of a fuel cell. The reservoir layer is a porous hydrophilic material. Excess liquid volume wicks into the reservoir layer through the catalyst layer and fills the smaller pores within the reservoir. The larger pores remain empty and provide clear passageways for the reactant gas to reach the catalyst. Such an embodiment requires precise control over the production of the reservoir layer to insure that the reservoir layer has the proper number and positioning of small and large pores. This again adds to the cost of the production of the fuel cell.
Accordingly, there is a need for a fuel cell having an electrolyte inventory control volume which is capable of being manufactured easily and inexpensively.